esp: drop unneeded assignment in esp4_gro_receive()

Making '!=' operation with 0 directly after calling
the function xfrm_parse_spi() is more efficient,
assignment to err is redundant.

Eliminate the following clang_analyzer warning:
net/ipv4/esp4_offload.c:41:7: warning: Although the value stored to
'err' is used in the enclosing expression, the value is never actually
read from 'err'

No functional change, only more efficient.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>